Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Dec. 31, 2012 Segment	Dec. 31, 2011	Sep. 30, 2012
Segment Reporting [Abstract]
Number of reportable segments (in segments)	2
Segment Reporting Information [Line Items]
Revenues	$ 273,797	$ 280,641
Cost of sales	138,277	156,906
Depreciation and amortization	13,571	13,063
Operating income	72,564	64,605
Interest expense	10,077	10,607
Income before income taxes	62,487	53,998
Total assets (at period end)	2,319,704	2,238,458	2,223,179
Goodwill (at period end)	182,145	182,145	182,145
Capital expenditures	29,728	22,858
Gas Utility [Member]
Segment Reporting Information [Line Items]
Revenues	248,254	255,030
Cost of sales	123,574	141,679
Depreciation and amortization	12,605	12,145
Operating income	69,789	61,231
Interest expense	9,572	10,098
Income before income taxes	60,217	51,133
Total assets (at period end)	2,164,368	2,088,710
Goodwill (at period end)	182,145	182,145
Capital expenditures	28,519	21,812
Electric Utility [Member]
Segment Reporting Information [Line Items]
Revenues	25,118	25,169
Cost of sales	14,703	15,227
Depreciation and amortization	966	918
Operating income	2,624	3,230
Interest expense	505	509
Income before income taxes	2,119	2,721
Total assets (at period end)	155,336	149,748
Goodwill (at period end)	0	0
Capital expenditures	1,209	1,046
Other [Member]
Segment Reporting Information [Line Items]
Revenues	425	442
Cost of sales	0	0
Depreciation and amortization	0	0
Operating income	151	144
Interest expense	0	0
Income before income taxes	151	144
Total assets (at period end)	0	0
Goodwill (at period end)	0	0
Capital expenditures	$ 0	$ 0